Last time I used white paper on the left and right of the glass. The rims get their lights from the left and right as the background and the paper on the left and right are not entirely together.
If you want to do it properly check out this picture.
SB-900 with diffusion dome behind black paper, -0.3EV, camera -2.7EV and @f11 to eliminate the rest of the ambient light. Triggered with the camera's internal flash.